Increased optical signal speed is conventionally demanded of optical transmission systems and various modulation schemes therefor are employed. One modulation scheme is, for example, differential quadrature phase shift keying (DQPSK) modulation. Dual polarization-quadrature phase shift keying (DP-QPSK) modulation and quadrature amplitude modulation (QAM) modulation can also be given as the modulation schemes.
Various multiplexing schemes to acquire multivalued optical signals are also employed such as wavelength multiplexing and polarization multiplexing. A technique of executing time-division multiplexing of optical signals is also present. For example, according to a known technique, an input light beam is divided into two clock pulses that are data-modulated and coupled, whereby dual division multiplexing is executed (see, e.g., Japanese Laid-Open Patent publication No. H2-167524).
However, according to the conventional techniques, the dual division multiplexing is the limiting factor and therefore, a problem arises in that no improvement of the transmission speed can be facilitated without the use of a high speed electronic circuit.